Dies for extruding thermoplastic material typically include a pair of die portions joined together to define an outlet through which molten thermoplastic material is extruded. Specifically, each die portion includes a lip, and the lips of the two die portions cooperate to define a die outlet or lip gap.
In one type of prior art extrusion apparatus, the lips are integral with the die portions and a first one of the lips is joined to a respective die portion by a flexible hinge. Apparatus is provided to adjust the position of the first lip and thus the width of the lip gap. Specifically, the adjustment apparatus includes a plurality of thermally responsive members spaced along the width of the die and a plurality of heater elements each disposed in heat-transfer relationship with a thermally responsive member. The heater elements are individually controlled to adjust the profile of the lip gap at one edge thereof. Alternatively, a manually operable adjustment apparatus may be provided to adjust the width of the lip gap. In either case, the adjustability of the die is limited by the deflection limit of the flexible hinge. Also, the initial set-up procedure to preset the desired outlet width can be undesirably time consuming.
In another prior art extrusion die, referred to as a "sliding lip" die, first and second die lip members are separate from and carried by respective die portions. The first die lip member is rigidly secured to the die body while a second die lip member is mounted for sliding movement relative to the first die lip member. Again, adjustment apparatus is provided for positioning the second die lip member so that a desired lip gap width is achieved and maintained. While this die design has a wide adjustment range, changing of the gap width can be a lengthy process and gauge control capability is limited.
A third prior art extrusion die includes first and second die body portions, one having a first lip coupled integrally therewith by a flexible hinge and the other having a second, removable lip bolted thereto. A plurality of thermally responsive members are coupled to the first lip for adjusting the position of that lip to achieve a desired extrudate profile. The removable lip, on the other hand, is stationary but can be replaced by other lip members having different dimensions so that the base lip gap can be selected as desired or necessary. This die has relatively good gauge control compared with other prior art die designs but requires a longer time to change the base lip gap and thus has a longer product changeover time.
Because processors of extruded film, sheet, and coatings often must meet their customers' demands with "just-in-time" deliveries, it is important that the time required to adjust the lip gap dimensions of extrusion dies be kept as short as possible to maximize production of extruded materials and minimize production of scrap material during changeovers. Moreover, because extruded materials are often required in a wide range of thicknesses, it is desirable to provide a single die having a wide adjustment range so that a variety of material thicknesses can be produced without the need to replace the lips or other components of the die and requiring minimal reconfiguration time.